1. Field of the Invention
The present invention relates to a numerical controller capable of setting an allowable amount of inward rounding at a corner arising between blocks. More specifically, the present invention relates to a numerical controller having the function to insert, between the two blocks forming the corner, a plurality of cubic or higher-order polynomial curves continuous in position, direction and curvature and no larger in inward rounding than a preset allowable amount of inward rounding.
2. Description of the Related Art
In numerical control for controlling a machine tool, a corner is formed in a machining path as the result of a change in the direction of movement, when blocks commanding machining are executed continuously. Shock is liable to occur in the machine tool since the velocity of each axis, including a moving axis, suddenly changes at the corner.
In general numerical control, acceleration and deceleration is performed in order to suppress such shock. Examples of acceleration and deceleration methods include pre-interpolation acceleration and deceleration in which acceleration and deceleration is performed along a machining path before interpolation processing and post-interpolation acceleration and deceleration in which acceleration and deceleration is performed on each axis after interpolation processing.
In pre-interpolation acceleration and deceleration, a calculation is made of such a corner velocity as to make the amount of velocity change in each axis no larger than a preset allowable velocity difference, in order to suppress a sudden change in the velocity of each axis at the corner. Velocity control is performed in a manner such that a feed rate is decelerated from a position a distance short of the corner in order that the feed rate at the corner agrees with the calculated corner velocity and is then accelerated on and after arrival at the corner.
In post-interpolation acceleration and deceleration, control is performed so as to locally average the velocity of each axis determined by pre-interpolation acceleration and deceleration on the basis of time, i.e., so as to suppress a velocity change in each axis, in order to further suppress shock arising in the machine tool. As a result, acceleration and deceleration is performed in an overlapping manner between blocks, and therefore, the machining path deviates from a commanded machining path, thus causing an inward rounding error.
The amount of inward rounding varies depending on the difference of a rounding angle at the corner, a difference in the moving axis between positions preceding and following the corner, or a time constant representing the characteristics of post-interpolation acceleration and deceleration. Accordingly, in order to keep the amount of inward rounding no larger than a given value, the allowable velocity difference of each axis or the time constant of post-interpolation acceleration and deceleration needs to be adjusted for each machine or each machining program used.
Japanese Patent Application Laid-Open No. 9-190211 discloses a technique to interpolate a curve into a corner in a case where acceleration is discontinuous at the corner and make a velocity and an acceleration continuous at both ends of the inserted curve. Six unknown quantities are required in order to make a position, a velocity and an acceleration continuous at both ends of the curve. In this patent document, this requirement is satisfied by a single quintic curve.
In addition, Japanese Patent Application Laid-Open No. 10-320026 describes an example of inserting a plurality of polynomial curves into a corner. If the curves to be inserted cubic curves, a direction and a curvature can be made continuous at both ends of the curves by inserting four cubic curves.
In the technique disclosed in Japanese Patent Application Laid-Open No. 9-190211 mentioned above, interpolating higher-order curves into the corner tends to complicate a path, resulting in difficulty in control. Complicated curves lead to a large curvature at the corner. This patent document does not specifically describe the way of determining such curves as to make a deviation from an original commanded path to fall within an allowable tolerance.
Three cubic curves will suffice for an original direction and curvature to be continuous. Unfortunately, however, such constraints as to make a deviation fall within the allowable tolerance at the midway point of curves to be inserted are added in Japanese Patent Application Laid-Open No. 10-320026 mentioned above, as a result, creating four cubic curves is needed.